The good news: a full English is the strongest start to your day (The bad news? You must work it off - or it's back to porridge)

If you find yourself reaching for the biscuit tin by mid-morning, the chances are your choice of breakfast is to blame. But, for those who do manage more than a cup of coffee before rushing out of the house, just what should the correct start to the day contain?

According to experts, a true power-breakfast should be made up of energy-giving carbohydrates, protein and fibre in the right proportions to maintain blood-sugar levels until early afternoon. The wrong combination, however, will mean wince-inducing hunger pangs before lunch is even a dot on the horizon.

The key, explains Catherine Collins, principal dietician at St George's Hospital, London, is in the Glycemic Index (GI) score of the meal.

GI Joe: Peter Knight sets out on his power-breakfast test to gauge the effects of the all-important Glycemic Index

'Some breakfasts, such as marmalade on toast, have a high GI score, which means they release sugar, burned as fuel for energy in the body, into your bloodstream quickly,' she says.

'This means you will feel hungry fairly quickly after eating. But porridge, for instance, has a very low GI score, meaning it is digested slower which makes you feel fuller for longer. This aids concentration and staves off cravings for fat and sugar-laden snacks.'

While GI scores are easily calculated for individual ingredients, it is not so simple for whole meals or combinations - for example, Weetabix and milk, says Collins.

'There is little point calculating an average GI for a meal because different foods have a different effect on each other when mixed in the stomach.'

So to find out which breakfast was best for giving sustained energy, I ate a different first meal at 8am each day for a week, and took blood samples every half an hour after, for four hours, using a monitor which analyses a pinprick sample of blood taken from the fingertip to measure the amount of glucose in the bloodstream.

To keep the test fair I worked from home, drank a cup of tea (one sugar) after every breakfast and avoided energy-sapping activities such as running or cycling.

Firstly, what works for me may not work for you - different body shapes have different rates of digestion. Secondly, the type of sugar found in fruit and honey (fructose) goes 'under the radar' of the test kit. So the fruit and yogurt breakfast that I tried will give a higher amount of sugar than the meter recorded.

Nevertheless, my fingertips have revealed some surprising results - and here Collins also gives her verdict on each breakfast.

Catherine's verdict: The marmalade is essentially sugar, which means it is quickly absorbed into the bloodstream. This will leave you hungry from mid-morning onwards because you have already digested the entire carbohydrate content of the breakfast early on.

Multigrain brown bread is no different to wholemeal bread or even white bread in terms of slowing digestion. Oat bread is the preferred choice as the oats are baked into the bread, creating a jelly-type fibre that takes the stomach longer to break down. Marmalade is high in sugar but it is still only 10g per two slices of bread, not enough to have a dramatic effect on the body's glucose level.

Peter's verdict: After two foodless hours my stomach was gnawing with discomfort. My blood-sugar readings were surprisingly high but I would have needed a snack soon after this breakfast.

Catherine: Weetabix is a medium GI food due to its gloopy, viscous texture when combined with milk. The stomach incubates the thick paste for longer, giving its acids more time to break it down. However, milk adds another 12g of sugar, which in theory would enter the bloodstream quicker.

So the rate at which the body uses this breakfast for energy depends on how milky you like your Weetabix.

A thicker mush with less milk would have kept Peter's hunger at bay for longer. To sweeten, add fruit such as strawberries or raisins - this is also an instant fuel supply.

Peter: I felt sated for three hours and, despite the lack of sugar, they were surprisingly tasty. The added bonus is that if I did want a sweet snack, because Weetabix is such a saintly cereal I would not feel so guilty.

Catherine: This contains about a tablespoon of sugar - by no means the highest sugar content breakfast - but it has the least amount of fat.

Fat slows down the rate at which sugar enters the bloodstream due to the stomach taking longer to break down the complex chain links that are in fat molecules. With little fat in the yogurt, the sugar from the fruit is absorbed into the blood quickly, meaning hunger will soon return.

On the plus side, yogurt is a great source of calcium, providing about a third of your daily needs, and the fruit salad counts for two of your five-a-day.

Peter: Two hours in and my stomach was growling, and I would have snacked if this wasn't a test.

The only merit this breakfast had was its lack of preparation but, overall, it was a poor start to the day.

Mark out of 10: 2

THURSDAY

Two croissants with butter and strawberry jam plus half-pint glass of orange juice.

Catherine: The blood glucose levels rose steadily as the jam and orange juice are rich in sugar. This would have been quickly absorbed into the bloodstream if not for the large amount of fat in the buttery pastry, which slows the digestive process.

More than half the carbohydrates here are sugar - which is not good for the waistline when the high fat and saturated fat content is also taken into account. This breakfast contains more calories than a Big Mac and fries so should be only an occasional treat.

Peter: Delicious as it was, this was more style over substance.

Three hours later and there were niggling hunger pangs affecting my concentration.

Catherine: This has the lowest sugar content of all the breakfasts so is a good test of how quickly the digestive system converts sugar to energy from starchy bread.

In this case, the rate at which sugar is released into the body is not particularly fast because although the multigrain is a high GI food, its digestion is slowed by the moderate amount of fat in the egg yolks and butter.

There is also no pure sugar (such as from jam or orange juice) that can be readily absorbed, so you should feel full for most of the morning. Eggs are carb-free and protein-rich, while lutein, a carotenoid found in egg yolk, will help the function of your eyes.

Peter: I managed a good four hours of work in the morning without giving in to snacks. My energy levels were reasonably constant.

Mark out of 10: 7

SATURDAY

Porridge (Waitrose Essential) with a tablespoon of honey and semi-skimmed milk.

Catherine: This meal is virtually sugar-free but constitutes nearly half of a man's recommended daily intake of calories.

The amount of fat here is the equivalent of half a shallow tub of margarine or 33 bowls of cereal - this could quite fairly be called a heart attack on a plate. But the fat slows the uptake of released sugar from the carbohydrates, meaning you should feel full for most of the morning.

A late rise in Peter's blood-sugar levels could have been caused by excess protein from the sausages and eggs being turned into glucose.

The choice of oil is a healthy one, but unless you are about to put in a couple of hours on a building site, I would suggest that this is a weekend treat best eaten as a brunch.

Peter: I loved this breakfast. However, as an energy booster it got off to a slow start. Indeed, it even made me sleepy.

But the energy readings show that this finished the strongest. Indeed, I did not even think about lunch for a further hour after the test had ended.